Silicas, particularly pyrogenic silicas, are characterized by their extremely finely divided state and their correspondingly high specific surface, by their very high purity, by their spherical particles and by the absence of pores. Due to these properties, there is an increasing interest in pyrogenic silicas as supports for catalysts (D. Koth, H. Ferch, Chem. Ing. Techn. 52, 628 (1980)).
It is known from DE-B 21 00 778 that granular materials based on pyrogenic silicas can be used as catalyst supports for the production of vinyl acetate monomer.
It is known from DE-A 38 03 900 that cylindrical particles which have arched end faces and which are based on pyrogenic silicas can be used as catalyst supports for the production of vinyl acetate monomer.
A process for the production of pressed parts is known from DE-A 39 12 504 in which aluminum stearate, magnesium stearate and/or graphite are used as a lubricant, and in which urea as well as methyl cellulose are used as pore forming agents.
These known pressed parts which are produced with magnesium stearate are commercially available as Aerosil Tablets No. 350, as supplied by Degussa. They contain about 0.4% by weight Mg.
Catalyst supports for catalysts for the synthesis of vinyl acetate monomer are known from EP 0 004 079 which consist of extruded sections with a star-shaped cross-section or which consist of ribbed lengths.
Catalysts for the synthesis of vinyl acetate monomer, which comprise at least one passageway channel with an inside diameter of at least 1 mm, are known from EP-B 464 633.
DE-A 195 38 799 describes a catalyst support in the shape of a honeycomb which predominantly consists of SiO.sub.2. According to Example 1 of said patent, this support has a diameter of 25 mm, a stay width of 1 mm, a stay spacing of 2 mm, and a length of 150 mm. After being coated with catalytically active elements, the resulting catalyst can be used for the production of unsaturated esters from olefins, acids and oxygen in the gas phase, for the purification of off-gas contaminated by organic substances, and for the alkylation of aromatic compounds.
WO 97/36679 also describes a catalyst support in the shape of a honeycomb, which is coated with SiO.sub.2 and which, after impregnation with palladium and gold and after activation with potassium acetate, can be used for the production of unsaturated esters.
Honeycomb-shaped catalysts are characterized by a very low pressure drop. However, the use of honeycomb-shaped catalysts in industrial reactors, particularly in tube bundle reactors, results in problems which are not inconsiderable, particularly with regard to packing tube reactors. With tube reactors, it is sometimes necessary to pack several thousand tubes of an industrial installation with honeycomb catalysts. In the course of this procedure, it has to be ensured that the honeycomb bodies do not break down during filling. Considerable emphasis has to be placed on the avoidance of edge flow effects, since otherwise the catalysts are not capable of contributing their full effect. Moreover, honeycomb-shaped catalyst materials exhibit poor thermal condutivity in a radial direction. This is particularly disadvantageous in reactions in which there is considerable evolution of heat, as in oxidation reactions for example. For the aforementioned reasons, there is currently no known industrial application in which a tube bundle reactor or thousands of tubes are operated with honeycomb-shaped catalyst materials. For this reason, reactors are packed with moldings in the form of pellets, which likewise exhibit a low pressure drop.
It is known from EP-B 0 519 435 that SiO.sub.2 can be pressed by means of binders to produce supports, followed by calcining the supports obtained and washing the calcined support particles with acid until cations from the binder are no longer released. In addition, a supported catalyst, a process for the production thereof, and the use thereof for the production of vinyl acetate are also described.
EP-A 0 807 615 describes pressed parts based on pyrogenic silica. These pressed parts can be used as a catalyst or catalyst support for the production of vinyl acetate monomer and for the hydration of ethylene. The pressed parts may be of different shapes, e.g. cylindrical, spherical or annular, with an outside diameter of 0.8 to 20 mm.